Espruino pico vbat power mode

[espruino-discuss]

Posted at 2018-07-18 by user92141

Hi!

I'm looking for a small stm32f4 developement board (even smaller than the stm32 blue pill) and found the espruino pico looking very promising.

But one of the requirements I have is to be able to enter VBAT mode and use the internal RTC calendar with low speed external clock (LSE). I have found that the pico supports run, sleep and stop mode (and standby mode if I code it myself) but nothing about vbat mode. To enter vbat mode on a stm32 the VDD must be powered off and VBAT powered on, I thought vbat mode was supported when I saw the vbat pins on the board but it seems like it is just for powering the board instead of USB. Is it possible to do some easy fix so I can enter vbat mode and use the internal RTC calendar?

I have read that i must solder on a crystal myself and maybe two capacitors for the LSE or use the espruino wifi board which has it soldered on from production.

Thanks in advance!

[gfwilliams]

Posted at 2018-07-19 by @gfwilliams

On the Pico and WiFi boards, the actual VBat pin in the STM32F4 is connected straight to VDD - I'm afraid there is no easy way to power just that and not VDD as well.

There are schematics available (linked from the Pico/WiFi pages) so you could potentially cut the track and wire up VBAT separately - but it would be very difficult

However you might find that entering the proper standby mode of STM32 gives you the power requirements you need. It's probably around 2 uA vs 1 uA for vbat mode - by comparison the MCP1703T voltage regulator on board draws 2 uA, so you're not going to see a massive difference in overall consumption between the two modes.